A phosphor powder of small particle size, and a process for producing it are known from I. Matsubara et al., Materials Research Bulletin 35 (2000), pages 217–224. The phosphor particles have a mean phosphor particle size of at least 1 μm. The phosphor particles consist of an yttrium aluminum garnet (Y3Al5O12) doped with chromium. Chromium is contained, for example, at 0.5 mol %. In this case, chromium constitutes an optically active component of the garnet. Chromium absorbs exciting light and emits emission light (luminescence) after excitement. The chromium contained in the garnet can also be excited to luminescence with the aid of electrons. Thus, the known phosphor powder is used, for example, in a luminescent screen (phosphor body) of a cathode ray tube. WO 03/102113 discloses a phosphor powder with a particle size of between 0.1 and 5 μm, which is used for a conversion LED.
The production of the known phosphor is performed with the aid of a so-called heterogeneous precipitation. For this purpose, aluminum sulfate (Al2(SO4)3) and urea are dissolved in distilled water. The solution is stirred continuously at a temperature of 80–90° C. for a duration of two hours. The precipitate of aluminum hydroxide (Al(OH)3) is formed in the process. The precipitate obtained is washed with distilled water and isopropanol and dried for one day at a temperature of 120° C. The aluminum hydroxide is suspended in distilled water. In order to avoid an agglomeration of the aluminum hydroxide powder, energetic stirring is carried out and urea is added. Stoichiometric quantities of yttrium sulfate (Y2(SO4)3) and chromium sulfate (Cr2(SO4)3) are likewise dissolved in distilled water. Thereafter, the suspensions and the solution are mixed with one another and heated for an hour to 80–90° C. The precipitate thereby obtained is washed with distilled water and isopropanol, centrifuged and dried overnight at 120° C. Finally, the powder obtained is calcined for two hours at 900° C.–1700° C. in the presence of air. A chromium-doped yttrium aluminum garnet is obtained that has a relatively high luminescent efficiency.
Since the phosphor powder produced using this process consists of phosphor particles that in each case have a layer that is a few tenths of a μm thick and does not contribute to the luminescence (dead layer), the phosphor particles must have a mean phosphor particle size of at least 1 μm for the high luminescent efficiency. A typical diameter of the phosphor particles is 1 μm on average. This means, however, that production parameters of the process must be set very accurately so that the resulting phosphor powder has the high luminescent efficiency.